This invention relates to a molding tool for manufacturing cup-shaped containers by deep-drawing a heated thermoplastic synthetic foil. The molding tool comprises an upper portion and a lower portion which are displaceable relative to one another. The lower portion constitutes the mold proper and is provided with a cutting edge. The upper portion includes a matrix and a spring-loaded depressor. A molding tool of the above-outlined type may be incorporated in the molding station of a thermoforming machine. The mode of operation of such a molding tool is conventional and is described, for example, in German Laid-Open Application (Offenlegungsschrift) No. 2,145,250.
For increasing the output of the molding tool it has been proposed to cool the depressor. Such a cooling may be effected by providing channels in the matrix and passing a coolant therethrough. Such a cooling, however, was not found to be thorough enough and resulted only in a slight output increase. According to another solution, as disclosed in United States patent application to Kiefer, Ser. No. 243,148, filed Mar. 12th, 1981, the depressor is exposed directly to a coolant flow. While such a solution has resulted in a substantial output increase, the provision of an appropriate seal between the coolant and the mold cavity has presented a significant problem. Since the seals are dynamically stressed in cycles of about 20 to 40 per minute, such seals wear relatively rapidly. A further factor which contributes to the wear of the seals is the fact that minute plastic particles present as a result of the stamping operation during the manufacturing process of the articles, may gain access to the clearance between the depressor and the matrix and thus contribute to the deterioration of the seal. Because of the limited space available, usually only O-rings or rings of quadratic cross section can be used; these are only conditionally adapted for dynamic stressing. Lip rings or similar, better adapted sealing elements have substantially greater dimensions and can therefore not be accommodated without introducing other disadvantages.
The immediate result of seal wear are leaks whereby coolant may gain access to the mold cavities. Such an occurrence is to be avoided at all cost, particularly taking into consideration the fact that the coolant often contains anti-corrosion additives and the articles to be manufactured are frequently intended for use as containers for foodstuff or as drinking cups.